As the size of semiconductor devices continues to decrease in order to improve the degree of integration, a resistor-capacitor (RC) delay related to wires of a circuit becomes a limiting factor affecting the speed of semiconductor devices. To reduce the RC delay, copper, which has a relatively low resistance, is used as a wire material, and a low-k material is used in an interlayer dielectric.
To use low-k materials, development of a novel material and application for a device are needed and, thus, may cost a large amount of money. Furthermore, even if a low-k material is used, effective k may increase, taking into consideration impacts of materials used in an etch stop layer or a barrier layer. Moreover, a low-k material typically has poor thermal stability and, therefore, properties of the low-k material may be modified by heat applied during formation of polysilicon used as a mask material in a double patterning process for forming bit lines, resulting in high dielectric constant.
Recently, a process of introducing an air gap into inter metal dielectrics (IMD) between wire lines has been used. Since air has a dielectric constant of 1.0, IMD having an air gap has a small effective k. FIG. 1 is a cross-section illustrating conventional formation of an air gap in a multi-layer wiring structure. As illustrated in FIG. 1, a first-layer copper pattern 20 and a second-layer copper pattern 40 are formed on a semiconductor substrate 10, and a copper line 30 having a narrower width than the widths of the first and second-layer copper patterns 20 and 40 is formed between the first-layer copper pattern 20 and the second-layer copper pattern 40. A low-k dielectric 50 is deposited using the second-layer copper wiring pattern 40 as a mask to form an air gap 55 below the second-layer copper pattern 40. Thus, to form an air gap, formation of a pattern having a greater width than that of a metal wiring line may be needed. However, In the case of bit lines, which do not constitute multi-layer structures, a mask pattern layer for forming an air gap may be needed.